Calf intestinal alkaline phosphatase (CIP) is a commonly used reporter enzyme for research and clinical assays. These assays typically use synthetic substrates that become detectable upon removal of phosphate groups by CIP. For example, one commonly used substrate is 1,2 dioxetane. This substrate becomes chemiluminescent upon the removal of phosphate groups by CIP. Another common substrate is the chromagen p-nitrophenylphosphate. Antibodies conjugated to CIP are widely used in ELISA, and nucleic acid probes linked to CIP can be used in a variety of nucleic acid detection schemes. Given the widespread use of CIP in various target detection schemes, it would be desirable to provide a class of ligands distinct from antibodies to which CIP could be conjugated.
The dogma for many years was that nucleic acids had primarily an informational role. Through a method known as Systematic Evolution of Ligands by EXponential enrichment, termed the SELEX process, it has become clear that nucleic acids have three dimensional structural diversity not unlike proteins. The SELEX process is a method for the in vitro evolution of nucleic acid molecules with highly specific binding to target molecules and is described in U.S. patent application Ser. No. 07/536,428, filed Jun. 11, 1990, entitled "Systematic Evolution of Ligands by EXponential Enrichment," now abandoned, U.S. Pat. No. 5,475,096 entitled "Nucleic Acid Ligand" and U.S. Pat. No. 5,270,163 (see also WO 91/19813) entitled "Nucleic Acid Ligands" each of which is specifically incorporated by reference herein in its entirety. Each of these applications, collectively referred to herein as the SELEX Patent Applications, describes a fundamentally novel method for making a nucleic acid ligand to any desired target molecule. The SELEX process provides a class of products which are referred to as nucleic acid ligands or aptamers, each having a unique sequence, and which have the property of binding specifically to a desired target compound or molecule. Each SELEX-identified nucleic acid ligand is a specific ligand of a given target compound or molecule. The SELEX process is based on the unique insight that nucleic acids have sufficient capacity for forming a variety of two- and three-dimensional structures and sufficient chemical versatility available within their monomers to act as ligands (form specific binding pairs) with virtually any chemical compound, whether monomeric or polymeric. Molecules of any size or composition can serve as targets in the SELEX method. The SELEX method applied to the application of high affinity binding involves selection from a mixture of candidate oligonucleotides and step-wise iterations of binding, partitioning and amplification, using the same general selection scheme, to achieve virtually any desired criterion of binding affinity and selectivity. Starting from a mixture of nucleic acids, preferably comprising a segment of randomized sequence, the SELEX method includes steps of contacting the mixture with the target under conditions favorable for binding, partitioning unbound nucleic acids from those nucleic acids which have bound specifically to target molecules, dissociating the nucleic acid-target complexes, amplifying the nucleic acids dissociated from the nucleic acid-target complexes to yield a ligand-enriched mixture of nucleic acids, then reiterating the steps of binding, partitioning, dissociating and amplifying through as many cycles as desired to yield highly specific high affinity nucleic acid ligands to the target molecule.
It has been recognized by the present inventors that the SELEX method demonstrates that nucleic acids as chemical compounds can form a wide array of shapes, sizes and configurations, and are capable of a far broader repertoire of binding and other functions than those displayed by nucleic acids in biological systems.
The basic SELEX method has been modified to achieve a number of specific objectives. For example, U.S. patent application Ser. No. 07/960,093, filed Oct. 14, 1992, now abandoned, and U.S. Pat. No. 5,707,796, both entitled "Method for Selecting Nucleic Acids on the Basis of Structure," now abandoned (see U.S. Pat. No. 5,707,796) describe the use of the SELEX process in conjunction with gel electrophoresis to select nucleic acid molecules with specific structural characteristics, such as bent DNA. U.S. patent application Ser. No. 08/123,935, filed Sep. 17, 1993, entitled "Photoselection of Nucleic Acid Ligands," now abandoned, U.S. Pat. No. 5,763,177 entitled "Systematic Evolution of Nucleic Acid Ligands by Exponential Enrichment: Photoselection of Nucleic Acid Ligands and Solution SELEX" and U.S. patent application Ser. No. 09/093,293, filed Jun. 8, 1998entitled "Systematic Evolution of Nucleic Acid Ligands by Exponential Enrichment: Photoselection of Nucleic Acid Ligands and Solution SELEX" describe a SELEX based method for selecting nucleic acid ligands containing photoreactive groups capable of binding and/or photocrosslinking to and/or photoinactivating a target molecule. U.S. Pat. No. 5,580,737 entitled "High-Affinity Nucleic Acid Ligands That Discriminate Between Theophylline and Caffeine," describes a method for identifying highly specific nucleic acid ligands able to discriminate between closely related molecules, which can be non-peptidic, termed Counter-SELEX. U.S. Pat. No. 5,567,588 entitled "Systematic Evolution of Ligands by EXponential Enrichment: Solution SELEX," describes a SELEX-based method which achieves highly efficient partitioning between oligonucleotides having high and low affinity for a target molecule.
The SELEX method encompasses the identification of high-affinity nucleic acid ligands containing modified nucleotides conferring improved characteristics on the ligand, such as improved in vivo stability or improved delivery characteristics. Examples of such modifications include chemical substitutions at the ribose and/or phosphate and/or base positions. SELEX process-identified nucleic acid ligands containing modified nucleotides are described in U.S. Pat. No. 5,660,985 entitled "High Affinity Nucleic Acid Ligands Containing Modified Nucleotides," that describes oligonucleotides containing nucleotide derivatives chemically modified at the 5- and 2'-positions of pyrimidines. U.S. Pat. No. 5,580,737, supra, describes highly specific nucleic acid ligands containing one or more nucleotides modified with 2'-amino (2'-NH.sub.2), 2'-fluoro (2'-F), and/or 2'-O-methyl (2'-OMe). U.S. patent application Ser. No. 08/264,029, filed Jun. 22, 1994, entitled "Novel Method of Preparation of Known and Novel 2' Modified Nucleosides by Intramolecular Nucleophilic Dispacement," now abandoned describes oligonucleotides containing various 2'-modified pyrimidines.
The SELEX method encompasses combining selected oligonucleotides with other and selected oligonucleotides and non-oligonucleotide functional units as described in U.S. Pat. No. 5,637,459 entitled "Systematic Evolution of Ligands by EXponential Enrichment: Chimeric SELEX," and U.S. Pat. No. 5,683,867 entitled "Systematic Evolution of Ligands by EXponential Enrichment: Blended SELEX," respectively. These applications allow the combination of the broad array of shapes and other properties, and the efficient amplification and replication properties, of oligonucleotides with the desirable properties of other molecules.
The SELEX method further encompasses combining selected nucleic acid ligands with lipophilic compounds or non-immunogenic, high molecular weight compounds in a diagnostic or therapeutic complex as described in U.S. patent application Ser. No. 08/434,465, filed May 4, 1995, entitled "Nucleic Acid Complexes"now U.S. Pat. No. 6,011,020 Each of the above described patents and applications which describe modifications of the basic SELEX procedure are specifically incorporated by reference herein in their entirety.
It is an object of the present invention to provide methods that can be used to identify nucleic acid ligands that bind with high specificity and affinity to calf intestinal phosphatase (CIP).
It is a further object of the present invention to obtain nucleic acid ligands to CIP that inhibit the activity of CIP when bound.
It is also an object of the present invention to obtain nucleic acid ligands to CIP that do not inhibit the phosphatase activity when bound to CIP.
An even further object of the invention is to provide a method for performing SELEX in a robotics-compatible microtiter plate format.
A still further object of the invention is to provide a method for absorbing SELEX targets to solid support surfaces, including microtiter plates, solely through hydrophobic interactions.